The invention concerns a sensor head for use in atomic force microscopy. The invention refers also to a method for the production of such sensor heads and to a method for the measurement of the deflection of the spring arm.
Such sensors heads are used in, among other things, atomic force microscopy (AFM), which is a very sensitive type of surface profilometry. The central component of a force microscope is the sensor head, which consists of a carrier element, a spring arm, and a sensor tip, which is swept over the specimen surface. The deflection of the spring arm caused thereby is detected with a suitable measuring method.
Force microscopy is used in two types of operations. In a measurement with atomic sensitivity, the tip is in contact with the surface of the specimen. The repulsive forces between the specimen and the first atom in the tip are thereby utilized. The contact forces of the spring arm lie in the range of 10.sup.-7 to 10.sup.-10 N. Spring constants of the spring arm of 0.01 to 10 N/m result therefrom.
Often even these slight forces are still too large and therefore result in deformations of the specimen surface. In these cases, for example, the attracting Van der Waals forces between the specimen and the tip are utilized. The tip is, moreover, in the interaction range of these forces, but is not in contact with the specimen surface. For the measurement, the spring arm is excited to resonant vibrations. With a change in the force gradient between the specimen and the tip, the effective spring constant of the system and thus also the resonant frequency change. Either this frequency shift is measured or, at a constant frequency, the change in the vibration amplitude of the spring arm caused thereby is detected with the aid of the lock-in technique. With this measurement method, an atomic resolution is not attainable. The resolution is, moreover, very greatly determined by the tip configuration, since many atoms of the tip contribute to the interaction.
In each of the two types of operations, measurements can be taken at a constant height or at a constant force or constant force gradient. In the first case, the distance between the spring arm and the specimen is maintained constant, and the deflection of the spring arm is recorded. In the second case, the force or the force gradient between the spring arm and the specimen is maintained constant by a servo loop. For this, the specimen is located, for example, on a piezoelectric adjusting element, with which the distance between the tip and the specimen can be adjusted.
Several methods are known for the detection of the deflection of the spring arm. The most accessible are optical detection methods, such as the light pointer principle and the interferometric principle.
From U.S. Pat. No. 5,017,010, the model of an interferometric force microscope is known. The spring arm is positioned up to a few microns before the end of a glass fiber, so that the light can exit from the fiber and can again be coupled into the fiber by the reflection at the spring arm. This light interferes with the light backreflected at the glass fiber end in the fiber, wherein the sinusoidal interference signal is used for the detection of the deflection of the spring arm. A high sensitivity is attained at the steepest points of the interference signal. The arrangement has the disadvantage, however, that this optimal working point must be adjusted mechanically. A piezoelectric adjusting element is used for this: it adjusts the distance between the end of the fiber and the microscope arm to the sensitive point of the interference signal. This system is therefore very expensive with respect to positioning. In addition, phase jitters of the light in the glass fiber has a drastic effect on the interference signal. For stability reasons, the glass fiber end is glued on and cannot be renewed without great adjustment expenditure. Since the system consists of different components, it has a very large thermal drift. The piezoelectric adjustment element is, moreover, an additional source of noise.
From European Patent 0,440,268 A2 and European Patent 0,290,648, the structure of a compact interferometric force microscope sensor is known, which has a spring arm attached via a hole. A partially transparent mirror is fastened to the lower side of the hole. The light arrives on the metal-deposited microscope arm by means of the partially transparent mirror and is reflected there. The reflected light interferes with the light reflected at the partially transparent mirror.
If the detection system is to attain the high resolution needed for force microscopy, the work must also be carried out at the steepest point of the interference signal. The disadvantage is that this point of the interference signal must be adjusted. This should be done through the electrostatic repulsion of the spring arm by utilizing the metal coating found on the spring arm and the carrier element as electrodes.
Another disadvantage is that the mirror must be positioned over the hole, which represents an additional manual adjustment expenditure. Moreover, the long optical path length in the sensor is disadvantageous since it requires a great coherence length of the light, so that the usable light sources are limited to lasers.
The most compact optical force microscope sensor until now consists of a laser diode feedback system, such as described in U.S. Pat. No. 5,025,658. A laser diode output signal is thereby formed with the spring arm by a Fabry-Perot interferometer, whose interference signal arrives back at a photodetector by means of the laser diode. In this system also, the most sensitive point of the interference signal is adjusted with a piezoelectric adjusting element. An atomic resolution was thus not attained, since the spring arm cannot work at the contact between the tip and specimen.
A force microscope is known from IBM Technical Disclosure Bulletin, Vol. 32, 1989, pages 416-417, in which the spring arm and distance spacer are made from one piece. This component is attached to a glass block, which is provided with a metal layer, as is the force arm, in order to be able to exert an electrostatic force on the force arm. It is not possible to set the desired distance to the carrier element without adjustment because of the manufacturing tolerances of such a one-piece component. In addition, there is also the fact that the one-piece component must be fastened to the glass block--for example, by means of an adhesive--which also leads to an undefined distance of the spring arm to the surface of the glass block.
The same is also true for West German Patent 4,107,605 C1, according to which even the sensing arm and the optoelectronic device form one common part and are made from one common plate. The entire arrangement is produced by etching. A readjustment is necessary also with this spring arm.
J. Vac. Sci. Technol. A, Vol. 8, 1990, pages 3386-95, describes two spring arms which are placed on a silicon element. The spring arm is exposed by etching. However, there is no carrier element which extends over the area of the sensor tip in the described arrangements.